Download Coulomb`s Law of Magnetism

Magnetism
CH 19
Magnetic Materials
• Materials that are attracted to magnets are
called ferromagnetic
• Substances that are repelled by magnets are
diamagnetic
• Substances that are slightly attracted by a
strong magnet are called paramagnetic
The Domain Theory of
Magnetism
• Magnetism is a property of a charge in
motion
• Magnetism is caused by the movement of
electrons
– An electron revolving around the nucleus of an
atom imparts a magnetic property to the atom
structure
– The electron spinning on its axis
• Each spinning electron acts as a tiny magnet
• Opposite spins are designated + and –
– Electrons spinning in opposite directions tend to form
pairs and so neutralize their magnetic character
• Domains
– Atoms are grouped in microscopic magnetic
regions called domains
– The atoms in each domain are magnetically
polarized parallel to a crystal axis
– The domains effectively cancel out one another
– If domains rotate with a external field, the
material becomes magnetized
– Curie Point – The temperature at which the
domain regions disappear and the material
becomes paramagnetic
Force Between Magnet Poles
• Like poles repel
• Unlike poles attract
• Unit pole – a pole that repels an exactly similar
pole, placed one centimeter away, with a force of
10-5 N
• Coulomb’s Law of Magnetism – the force of
attraction between two magnetic poles is directly
proportional to the product of the strengths of the
poles and inversely proportional to the square of
the distance between them
• F = k M1M2
d2
Magnetic Fields of Force
• Magnetic fields can be represented by lines
of flux
• A line of flux is drawn so that a tangent to it
at any point indicates the direction of the
magnetic field
• The unit of magnetic flux () is the weber
(Wb)
• The magnetic flux density (B) is the number
of flux lines per unit area that permeates the
magnetic field
• B = /A
• 1 weber/meter2 = 1 tesla
Magnetic Permeability
• The property of a material by which it
changes the flux density in a magnetic field
from the value in air is called its
permeability
– All materials that are not ferromagnetic behave
similar to air (they are magnetically inert)
• Magnetism produced in a ferromagnetic
substance by the influence of a magnetic
field is called induced magnetism
Terrestrial Magnetism
• The earth sort of behaves like it has a giant
bar magnet inside of it
• The earth’s magnetic axis does not coincide
with its geographic axis, it is inclined at a
small angle
– Magnetic N pole is located 1200 mi from N
pole
– So N pole of a compass needle does not point
to the true geographic north
Declination
• At any surface location the angle between
magnetic north and the true north is called
the declination, or variation
– LA = 15 degrees east
– Cincinnati, OH = 0 degrees
– Boston = 15 degrees west
Electromagnetism
• A magnetic field exists in the region near
the conductor when a closed circuit exists
• The direction of the magnetic field is
dependent on the direction of the current in
the conductor
– Example
Magnetic field and charge in
motion
• Ampere’s Work
– Two long, straight, parallel conductors of length l
separated by distance d and carrying currents I1 and I2
will each experience a force F of magnitude
F=2klI1I2/d
k = 10-7 N/A2
– Ampere – the current in each of two long parallel
conductors spaced one meter apart that causes a
magnetic force of 2 x 10-7 newton per meter length of
conductor
– Coulomb – the quantity of electric charge that passes a
given point on a conductor in one second when the
conductor carries a constant current of one ampere
Ampere’s Rule (Left hand rule)
• Grasp the conductor in the left hand with
the thumb extended in the direction of the
electron flow. The fingers will circle the
conductor in the direction of the magnetic
flux
The Galvanometer
• Used to detect the presence of an electric
current or to determine its direction
• Made of a coil of wire wound on an iron
core that is pivoted on jeweled bearings
between the poles of a permanent horseshoe
magnet
Other Instruments
• DC Voltmeter
– The dc voltmeter is basically a high-resistance
galvanometer, used to measure voltages
• DC Ammeter
– An ammeter is basically a modified
galvanometer used to measure current
– Must be wired in series
• Ohmmeter
– Measures the resistance of a circuit component
– Must be used only on a completely deenergized circuit